1. Field of the Invention
The present invention relates to image capturing apparatuses, light metering methods, luminance calculation methods, and programs, and, more particularly, to an image capturing apparatus capable of switching between a normal image capturing mode and a combined image capturing mode at the time of image capturing.
2. Description of the Related Art
In image capturing apparatuses in the related art using a solid-state image pickup device such as a CCD (Charge Coupled Device), the quantity of light to be input into the image pickup device (quantity of light exposure) is controlled by controlling an aperture and an electronic shutter speed. That is, when a bright scene is captured, the quantity of light exposure is reduced so as to prevent the occurrence of overexposure that is caused by saturation of a signal output from an image pickup device. Conversely, when a dark scene is captured, the quantity of light exposure is increased so as to prevent the occurrence of underexposure.
However, when a scene with a strong contrast is desired to be captured (image capturing of a backlit subject or simultaneous image capturing of an indoor scene and an outdoor scene) using a solid-state image pickup device with an insufficient dynamic range, even if the quantity of light exposure is controlled, overexposure may occur in a bright portion of the scene due to the saturation of the bright portion and underexposure may occur in a dark portion of the scene. As a result, both of these portions cannot be appropriately reproduced.
As a method of overcoming such a difficulty, Japanese Unexamined Patent Application Publication No. 6-141229 (corresponding U.S. Pat. No. 5,455,621) discloses a method of separately obtaining bright area information and dark area information by using two different electronic shutter speeds in a field or changing an electronic shutter speed for each field, and combining the obtained pieces of information into a single image. This method is employed by apparatuses capable of capturing an image with a wide dynamic range (wide dynamic range cameras). In addition, Japanese Unexamined Patent Application Publication Nos. 2002-84449, 2004-120205 (corresponding patent No. US 2006/033823 A1), and 2006-319474 disclose apparatuses and methods for improving the quality of the above-described combined image.
Japanese Unexamined Patent Application Publication Nos. 2002-84449 and 2004-120205 (corresponding patent No. US 2006/033823 A1) disclose a method of improving image quality of a wide dynamic range camera by improving the method of combining images captured with two significantly different quantities of light exposure, however, they do not describe a light metering method for exposure control.
Wide dynamic range cameras generally have two types of image capturing modes, a combined image capturing mode in which wide dynamic range image capturing is performed and a normal image capturing mode in which the wide dynamic range image capturing is not performed. In general, a light metering method automatically set in the normal image capturing mode (a camera user is not required to perform a setting operation) is a center-weighted metering method or an evaluative metering method. The center-weighted metering method is a method of measuring light concentrating on the center area of an image under the assumption that a main subject exists in the center area. For example, Japanese Unexamined Patent Application Publication No. 2006-319474 discloses this method. The evaluative metering method (this method is also called a multi-segment metering method and a multi-pattern metering method) is a method of dividing an image into a plurality of areas, measuring light in each of these areas, determining whether excessive forward light correction or backlight correction should be performed on the basis of the light measurement results of these areas, and performing appropriate exposure. In the evaluative metering method, an in-focus point is also sometimes taken into consideration, or the image is sometimes divided into a larger number of areas. Thus, an algorithm used for the evaluative metering method varies from camera manufacturer to camera manufacturer.
In order to perform appropriate exposure for a main subject, these two light metering methods in which weights are assigned to a particular part are used in the normal image capturing mode used for image capturing of an image with a narrow wide dynamic range. On the other hand, in the combined image capturing mode used for image capturing of an image with a wider dynamic range, subjects in the entire area of a captured image are required to be visually checked. Accordingly, the above-described light metering methods are not suitable for the combined image capturing mode.